1. Field of the Invention
The invention relates to a thermally-assisted magnetic recording head used in thermally-assisted magnetic recording in which near-field light is applied to a magnetic recording medium to lower a coercivity thereof so as to record information.
2. Description of Related Art
In the past, a magnetic disk unit has been used for writing and reading magnetic information (hereinafter, simply referred to as information). The magnetic disk unit includes, in the housing thereof for example, a magnetic disk in which information is stored, and a magnetic read write head that records information into the magnetic disk and reproduces information stored in the magnetic disk. The magnetic disk is supported by a rotary shaft of a spindle motor, which is fixed to the housing, and rotates around the rotary shaft. On the other hand, the magnetic read write head is formed on a side surface of a magnetic head slider provided on one end of a suspension, and includes a magnetic write element and a magnetic read element that have an air bearing surface (ABS) facing the magnetic disk. In particular, as the magnetic read element, an MR element exhibiting magnetoresistive effect (MR) is generally used. The other end of the suspension is attached to an end of an arm pivotally supported by a fixed shaft installed upright in the housing.
When the magnetic disk unit is not operated, namely, when the magnetic disk does not rotate and remains stationary, the magnetic read write head is not located over the magnetic disk and is pulled off to the outside (unload state). When the magnetic disk unit is driven and the magnetic disk starts to rotate, the magnetic read write head is changed to a state where the magnetic read write head is moved to a predetermined position over the magnetic disk together with the suspension (load state). When the rotation number of the magnetic disk reaches a predetermined number, the magnetic head slider is stabilized in a state of slightly floating over the surface of the magnetic disk due to the balance of positive pressure and negative pressure, and thus, information is accurately recorded and reproduced.
In recent years, along with a progress in higher recording density (higher capacity) of the magnetic disk, improvement in performance of the magnetic read write head and the magnetic disk has been demanded. The magnetic disk is a discontinuous medium including collected magnetic microparticles, and each magnetic microparticle has a single-domain structure. In the magnetic disk, one recording bit is configured of a plurality of magnetic microparticles. Since it is necessary for the asperity of a boundary between adjacent recording bits to be small in order to increase the recording density, it is necessary for the magnetic microparticles to be made small. However, if the magnetic microparticles are made small in size, thermal stability of the magnetization of the magnetic microparticles is disadvantageously lowered with decrease in volume of the magnetic microparticles. To solve the issue, increasing anisotropy energy of the magnetic microparticle is effective. However, increasing the anisotropy energy of the magnetic microparticle leads to increase in coercivity of the magnetic disk, and as a result, difficulty occurs in the information recording in the existing magnetic head.
As a method to solve the above-described difficulty, a so-called thermally-assisted magnetic recording has been proposed. In the method, a magnetic disk with large coercivity is used, and when information is written, heat is applied together with the magnetic field to a section of the magnetic disk where the information is to be written to increase the temperature and to lower the coercivity of that section, thereby writing the information. Hereinafter, the magnetic head used in the thermally-assisted magnetic recording is referred to as a thermally-assisted magnetic recording head.
In performing the thermally-assisted magnetic recording, near-field light is generally used for applying heat to a magnetic disk. For example, in Japanese Unexamined Patent Application Publication No. 2001-255254 and in Japanese Patent No. 4032689, disclosed is a technology of allowing frequency of light to coincide with a resonant frequency of plasmons that are generated in a metal, by directly applying the light to a plasmon generator, in order to generate near-field light. In the method of directly applying light to a plasmon generator, however, the plasmon generator itself overheats and accordingly deforms, depending on usage environment or conditions. Therefore, practical realization of the method is difficult.
Therefore, as a technology capable of avoiding such overheating, in Japanese Patent No. 4104584, a thermally-assisted head using surface plasmon polariton coupling is proposed. In this technology, without direct irradiation of light propagating through a waveguide (guided light) to a plasmon generator, the guided light is coupled to the plasmon generator through evanescent coupling, and surface plasmon polaritons generated on a surface of the plasmon generator are used.
In the thermally-assisted magnetic recording head using such surface plasmon polariton, temperature increase of the plasmon generator is suppressed to some extent. However, it was confirmed that, for example, when the plasmon generator is formed of Au (gold), shrinkage (agglomeration) due to heat may occur particularly in a section with small volume in the vicinity of the ABS where the heat is concentrated, in some cases.
Such agglomeration is considered as a phenomenon caused by the fact that gold configuring the plasmon generator is not in a stable state like a bulk state. Specifically, it is conceivable that this is because the density of gold formed by plating or sputtering is low so that own density is increased by temperature increase at the time of the thermally-assisted magnetic recording head being operated and the crystal structure proceeds in a stabilization direction.
Therefore, it is preferable that heat treatment be performed in advance in manufacturing phase to stabilize a crystal structure of a material (for example, gold) configuring the plasmon generator.
However, it is desirable to avoid the heat treatment at a temperature thermally damaging operation performance of an MR element because a thermally-assisted magnetic recording head is also provided with a magnetic read head including an MR element. Accordingly, it is virtually difficult to sufficiently stabilize the crystal structure of the material of the plasmon generator and to sufficiently suppress the agglomeration during operation.
Such agglomeration of the plasmon generator is promoted also by stress applied to the plasmon generator, as described in Takeshiro Nagai, Molecular Dynamics Simulation of Void Generation during Annealing of Copper Wiring, Materials Transactions, Vol. 50, No. 10 (2009) pp. 2373 to 2377. The stress applied to the plasmon generator is caused by stress of overcoat covering the thermally-assisted magnetic recording head or stress of a cladding layer adjacent to the plasmon generator. It is conceivable that such stress is generated by the fact that change in volume (expansion and shrinkage in volume) in an in-plane direction of the overcoat is suppressed by a base substance formed of Al.TiC or the like as a base provided with the thermally-assisted magnetic recording head. In the overcoat and the cladding layer, since the end surface thereof exposed on the ABS that is formed by lapping is opened, slight change in volume occurs. Therefore, the plasmon generator is in a state where large stress is applied.
Further, the temperature is largely increased during operation in a tip section of the plasmon generator, that is, in the vicinity of the air bearing surface, and thus stress caused by difference between a coefficient of thermal expansion of the metal configuring the ABS and a coefficient of thermal expansion of the surrounding dielectric body. It is conceivable that such thermal stress is also caused by the agglomeration of the plasmon generator.